Support mixync portable.musiclib @local (writing back into a mixxxdb)

[fwcd]

• This direction requires caution, since we have to be careful to not overwrite any data by the
• Perhaps create a backup of the user's mixxxdb before doing anything via this tool to a mixxxdb.sqlite3.backup file in the same folder (Mixxx's config directory)?
• Only copy music to folders explicitly managed by this tool (i.e. mixync should not touch any Music directories previously configured by the user in Mixxx and instead create its own)
• Add ability to add a musiclib's folders only by reference?

Progress:

• Add --dest-root-dir
• Implement directory mapping (i.e. absolutize_directory)
• Implement track mapping (i.e. absolutize_track)
• Implement metadata (track, playlist, crate, ...) updating in MixxxStore
• Add additional safety measures for writing back into a Mixxx database #16
• Test it in a isolated environment